Manually resettable safety lockout switch



Feb. 17, 1970 A. s. B. METCALF 3,496,509

MANUALLY RESETTABLE SAFETY LOCKOUT SWITCH Filed Dec. 2. 1966 Uited States Patent M 3,496,509 MANUALLY RESETTABLE SAFETY LOCKOUT SWITCH Arthur G. B. Metcalf, Winchester, Mass., assignor to Electronics Corporation of America, Cambridge, Mass., a corporation of Massachusetts Filed Dec. 2, 1966, Ser. No. 598,752 Int. Cl. H01h 71/16, 71/50, 73/22, 77/04 U.S. Cl. 337-72 6 Claims ABSTRACT OF THE DISCLOSURE A manually resettable safety lockout switch includes a housing having a projecting stud on which is slidably mounting a contact actuating sleeve to which is secured a first bimetallic latch member. A manually actuable push button moves the sleeve to a latched position where it is held by engagement of the first latch member with a second bimetallic latch member. A heater wire is wound over a sandwich of two mica sheets secured to the second latch member, and on energization of the heater wire for a predetermined interval, the latch is released.

This invention relates to manually resettable safety switches and more particularly to a manually resettable safety switch having a latch mechanism which is released in response to a signal from a supervising circuit.

Safety switches of the type to which the invention relates are employed to change the condition of one or more supervised circuits in response to the presence of some predetermined undesirable condition as indicated by a signal from a supervising circuit. Such a switch arrang ment finds particular use in a combustion supervision system in which it is desired to deenergize the control equipment upon a detected condition of flame failure in the supervised combustion chamber. Such a control device must operate reliably and in response to a signal of predetermined magnitude and/or duration to change the condition of the supervised circuits or components. Stringent specifications are established in many jurisdictions for the operating characteristics of such safety switches.

It is an object of this inventon to provide a novel and improved safety switch which provides reliable response to a supervising signal accurately and with precision.

Another object of the invention is to provide a novel and improved safety switch construction susceptible of economical manufacture.

A further object of the invention is to provide a novel and improved safety switch which incorporates an improved latching mechanism.

In accordance with the invention there is provided a manually resettable safety switch structure having a base housing on which is mounted a pair of electrical circuit contacts for connection in a supervised circuit. These contacts are movable between a closed condition and an open condition. The base housing also carries a contact actuator for controlling the condition of the contacts and a biasing structure which urges the actuator towards a first position in which the contacts are in one of said conditions. A manual operator structure is mounted on the housing for moving the actuator toa second position in which the contacts are placed in the other of said conditions. The switch structure further includes a latch structure that includes a first latch member secured to and movable with the actuator. The first latch member includes a thermally responsive strip portion. A cooperating latch member, also including a thermally responsive portion that is disposed parallel to the thermally responsive portion of the first latch member, has a lit) 3,496,509 Patented Feb. 17, 1970 latch surface that engages a cooperative surface of the first latch member in a latching operation. A heater structure mounted on thermally responsive portion and preferably on the cooperating latch member, includes terminals for connection to a supervising circuit. The latch sturcture, when the latch members are in latched condition, holds the contact actuator in the second position. Electrical energy applied to the heater structure generates heat which causes its coupled latch member to flex away from the other latch member in a latch release operation which frees the contact actuator for movement under the influence of biasing structure towards its first position.

In the preferred embodfiment the heater structure is in the form of a coil of resistance wire which is mounted on two thin sheets of insulating material secured on either side of the thermally responsive strip portion in a sandwich configuration. The resulting heater structure is simple and economical and yet is capable of easily being provided with accurate thermal characteristics coordinated with those of its supporting latch member, for example, either a delay of 79 seconds or a delay of 1215 seconds. The latch structure preferably further includes on the latching surface of at least one of the latch members a material such as polytetrafluoroethylene which has substantially the same static and dynamic friction characteristics to provide improved release characteristics of the latch structure. The switch structure is relatively simple to construct to precise tolerances and responds reliably and uniformly to signals from supervising circuits to latch out circuits that are supervised by it in response to such signals.

Other objects, features, and advantages of the invention will be seen as the following description of a particular embodiment of the invention progresses, in conjunction with the drawing in which:

FIG. 1 is a perspective view of a switch structure constructed in accordance with the invention;

FIG. 2 is a perspective view of the opposite side of the switch structure shown in FIGURE 1;

FIG. 3 is end view of the switch structure shown in FIGURES 1 and 2;

FIG. 4 is a plan view of the switch structure shown in FIGURE 1 showing details of the latch structure in released condition;

FIG. 5 is a sectional view taken along the line 5-5 of FIGURE 3 showing the position of components when the switch structure is in released condition;

FIG. 6 is a sectional view similar to that of FIGURE 5 showing the position of components when the switch structure is in latched condition;

FIG. 7 is a diagrammatic view taken generally along the line 77 of FIGURE 3 indicating the relationship of a second set of contacts and actuator when the switch is in released condition;

FIG. 8 is a diagrammatic view similar to FIGURE 7 indicating the second contact set and actuator position when the switch is in, latched condition; and

FIG. 9 is an exploded view of components of the latch structure.

With reference to FIGS. 1 and 2 the safety switch structure includes a base housing 10 of rectangular configuration having a manual operator in the form of pushbu-tton 12 and two sets of contacts, 14a, 14b and'1-6a,:16b, which are mounted on the housing for connection to external circuits that are supervised by the safety switch. A contact actuator structure 18 is movable by operator 12 to change the condition of the sets of contacts 14, 16.

On the opposite side of the switch housing 10 from the contacts sets 14, 16 is mounted a latch structure which includes latch member 20 connected to actuator 18 and a 3 cooperating latch member 22 secured to housing 10. Mounted on latch member 22 is a heater element in the form of a resistance wire 24 which has several turns disposed around electrical insulating structure 2-6 that consis-ts of the two sheets of mic-a 28, 30 secured together by rivets 32 in sandwich relation to latch member 22. The heater coil 24 is connected to terminals 34 for connec tion to a supervising circuit.

With reference to FIG. the base housing has a projecting stud portion 40 disposed generally in the center of the housing aligned with the axis of push-button '12. Stud portion 40 has at its upper end a stud projection 42 of reduced dimenisons that receives a sleeve port-ion 44 that depends from the body of the push-button 12. Spring 46, received within sleeve 44, engages the top of stud projection 42 and act to bias push-button structure 12 upwardly away from fixed stud portion 40. The pushbutton is prevented from-movement out of the housing 10 by lips 48 formed on the upper wall of the housing which engage the upper edges of the push-button body. The push-button body further includes a surface 50 that engages the upper end 52 of contact structure 16a and acts to flex that contact outwardly away from contact 16b when push-button 12 is depressed; and a surface 54 which engages the actuator 18.

Also mounted on the housing stud portion 40 is a sleeve portion 58 of actuator 18 which has a lip 60 at its upper end against which a spring 62, disposed within sleeve portion 58 is held with the lower end of spring 62 acting against the upper surface of the main body of stud portion 40. Actuator 18 has two actuator surfaces similar to push-button surface 50, a surface 64 (FIG. 5) which engage-s the upper end of 66 of contact 16b and a surface '68 (FIG. 7) which engages the upper end 70 of contact 1411.

Secured to actuator 18 is latch member 20 which is a 'bi-metal strip formed in U-configuration having one leg 90 extending parallel to and secured to a sleeve surface, a second leg 92 having a bifurcated end defined by resilient fingers 93, 94 and a bi-ght portion 95 (as indicated in FIG. 9) with the high expansion side of strip 90 on the inside of the high-t portion 95. Mounted on the bifurcated end of leg 92 is 'a cap member 96 of polytetrafluoroethylene, the smooth upper surface 97 of which forms a latching surface which cooperates with latch member 22.

Latch member 22 is a similar bi-metal strip having a major leg portion 98 that is disposed generally parallel to portion 92 and a terminal lip portion 100 which has a radiused end surface 101 that overlies and engages surface 97 of plastic cap 96 when the latch components are in latching engagement. The high expansion side is designated by the numeral 101a. Leg portion 98 is secured to base housing -10 by means of rivets 102 in holes 103.

Mounted on leg portion 98 are two mica sheets 28, 30, each 0.005 inch in thickness. Each sheet is of generally H-s'ha-ped configuration with recesses in opposite sides which define a receptacle for the coil of heating wire 24 that is formed in a coil of four turns about strip 98. The two mica sheets 28, 30 are secured together in sandwich relation with the metal strip 98 by rivets 32. Insulated lead 104 connects one end of heater coil 24 to one terminal 34 while insulated lead 106 connects the other end of the heater coil to the other terminal 34.

In use the switch is mounted with contacts 14a, 14b and 16a, 16b connected to supervised circuits and terminals 34 connected to a supervising cincuit. To place the switch supervised circuits in operation, push-button 12 is depressed and its surface 54 acts against the upper surface of actuator 18 driving that actuator down, compressing spring 62 so that the cap 96 of latch member 20 is below lip "100 of latch member 22 and the resilience of latch member 20 moves cap 96 outwardly into the latching position shown in FIGS. 5 and 7. In that switch condifit tion, the supervised circuit controlled by contact 16a and 16b is in closed condition and the supervised circuit controlled by contacts 14a, 14b is in open condition.

Should a signal be provided by the supervising circuit, current flows through the heater coil 24 imparting heat to latch member 22 which causes the bimetal strip portion 98 to flex outwardly and within a short and predetermineded period of time to move the latch lip off the latch surface 97 and release the actuator 18 so that the switch components move to the position shown in FIGS. 6 and 8, opening the circuit controlled by contacts 16a, 16b and closing the circuit control-led by contacts 14a, 14b. The frictional characteristics of the engaged latch surfaces are such that they are readily freed upon flexing of member 22. The pararneters of the heater coil 24 relating to strip 22 are easily and reproducibly established with precision through the sandwich type construction which has preestablished heat transfer characteristics. The time delay response is accurate but may be easily modified during manufacture to satisfy particular requirements.

Compensation for changes in ambient temperature is provided by the substantially parallel leg portions 92 and 98 of the latch members which are arranged so that the latch members are flexed in the same direction under changing ambient thermal conditions. The switch structure provides a reliable safety switch structure of precision response to a supervising signal in an assembly which is relatively easy to manufacture and maintain and which is of long operating life.

While a particular embodiment of the invention has been shown and described, various modifications thereof will be apparent to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodiment or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

What is claimed is:

1. A manually resettable switch structure comprising a base,

a pair of electrical circuit contacts mounted on said switch base for connection to a supervised circuit, said contacts having a closed condition completing said supervised circuit and an open condition interrupting said supervised circuit,

a contact actuator for controlling the condition of said contacts, said contact actuator being mounted for sliding movement relative to said switch base between a released position in which said contacts are in one of said conditions and a latched position in which said actuator urges said contact towards the other of said conditions,

biasing structure urging said contact actuator towards said released position,

a manual operator structure operative in response to depressing movement from a first position to a second position to engage and slide and contact actuator to said latched position, said operator structure including a surface engaging at least one of said contacts when in said second position for holding that contact in said one condition even though said actuator is in said latched position, biasing structure urging said operator structure toward said first position,

a latch structure including a first latch member secured to and slidable with said contact actuator, said first latch member including a first bimetal strip portion,

a cooperating latch member secured to said switch base and including a second bimetal strip portion disposed generally parallel to said first bimetal strip portion of said first latch member,

a plastic surface secured to one of said latch members, said plastic surface defining a smooth latch surface adapted to engage, in latching relation, a cooperating latch surface of the other latch member,

said smooth and cooperating latch surfaces having interrelated frictional characteristics that are substantially the same under static and dynamic conditions,

said cooperating latch surface engaging said smooth latch surface when said contact actuator is moved to said second position to hold said actuator in said second position,

and a heater structure mounted adjacent and in elec trical isolation from the bimetal strip portion of one of said latch members for connection to a supervising circuit, said heater structure, on energization thereof, heating said one latch member to flex said one latch member away from the other latch member to release said latch structure and allow said biasing structure to slide said contact actuator to said first position.

2. The apparatus as claimed in claim 1 wherein said heater structure includes two sheets of insulating material disposed in sandwich relation on opposite sides of said one latch member and a resistance wire wound in a coil about said pair of sheets.

3. The apparatus as claimed in claim 2 wherein each said sheet of insulating material is composed if mica.

4. The apparatus as claimed in claim 3 wherein each said mica sheet has a thickness of about 0.005 inch.

5. The apparatus as claimed in claim 1 wherein the latching surface of one of said latch members consists of polytetrafluoroethylene.

6. The apparatus as claimed in claim 5 wherein said plastic surface is formed on a cap frictionally secured to said one latch member.

References Cited UNITED STATES PATENTS FOREIGN PATENTS France.

OTHER REFERENCES Flom, D. G. and Porile, N. T.: Friction of Teflon Sliding on Teflon, Journal of Applied Physics, vol. 26, N0. 20 9, pp. 1088-89, September 1955.

BERNARD A. GILHEANY, Primary Examiner R. L. COHRS, Assistant Examiner U.S. Cl. X.R. 

